For several decades exothermic catalytic balance reactions in a gaseous phase have been conducted in synthesis reactors traditionally containing several layers of catalyst, between or among which were arranged the means of controlling and regulating the reaction temperatures. The cooling, either continuous within the catalytic compound or intermittent between layers, allows the reaction conditions to be separated from equilibrium conditions with resultant increase in the speed of the reaction.
The constraints imposed by economic conditions have driven research toward facilities whose purpose is to decrease the energy consumption of the synthesis loop or, more generally, the overall consumption of the installation manufacturing ammonia or methanol.
In particular, decreasing the pressure of the exothermic synthesis in the gaseous phase has been suggested, which leads to the construction of larger and larger installations.
Increasing the conversion rate has also been suggested. This rate can be increased by replacing the temperature control, done by quenching between catalytic beds, by one or more internal exchangers. The complicated mechanical design of such reactors causes a high expense and uncertain reliability, and the operations for loading and emptying of the catalyst are made more difficult. The most up-to-date multilayer reactors do not easily lend themselves to the introduction of one or more exchangers between the layers in addition to the outlet exchanger because of complexity and risk of internal leaks.
The conversion rate has also been increased by decreasing the particle size of the catalyst, which increases its activity but also its pressure drop.
The conversion rate has also been increased by increasing the volume of the reactor. Vertical reactors with axial circulation allow this to happen only to a limited extent, their pressure drop becoming prohibitive beyond about 70 m.sup.3 of catalyst. Vertical reactors with radial circulation as well as horizontal multilayer reactors with vertical circulation allow an increase in the volume of the catalyst by elongating the container without increasing the diameter. Radial multilayer reactors as well as many axial reactors present disadvantages involving reliability and risk of leaks due to an opening equal to the diameter of the pressure casing for introducing and withdrawing the internal equipment while the catalyst is being changed. It is difficult to make the wide-diameter flanges tight. The diameter is, in general, limited by that of the opening because of structural problems and the permissible size for transport. Furthermore, increasing the volume of the catalyst in multilayer radial and horizontal reactors leads to very long internal equipment, resulting in problems in construction, reliability and changing the catalyst.
Another disadvantage of multilayer reactors lies in the fact that their rate of filling: the ratio of the volume of the catalyst to the volume of the enclosed space under high pressure, minus the volume of the internal exchangers, is only about 0.5.
The development of technologies and economic conditions in the sector of mechanical design tends to make complex internal equipment made of stainless steel very expensive and, in contrast, the simple technique of so-called single layer containers, in series, is competitive. The latter uses only a large reactor but large or small vertical reactors containing a single layer of catalyst, in series, and connected to each other by piping, the exchangers being enclosed in separate containers. This technique does not present all the disadvantages of multilayer reactors, but it does not easily lend itself to using large volumes of catalyst since the pressure drop increases rapidly due to the axial circulation of the gases. On the contrary, the single-layer horizontal reactors with vertical circulation are much better for use with large catalylic volumes.
As of 1944, at a time when ammonia was produced in low volume catalytic facilities, German Pat. No. 911,490 already proposed a horizontal reactor containing a single layer of catalyst in which the gases under high pressure and temperature of the reaction are in contact with the pressure casing. These operating conditions require the use of highly alloyed steels which are very difficult to use in wide diameters. This type of reactor is not transferrable to large synthesis units.
More recently, French Pat. No. 2,183,985 described a horizontal reactor for synthesizing ammonia with an internal-external vertical heat exchanger and two reaction chambers with catalyst layers. This vertical exchanger provides a heat exchange between the gas streams. This solution appears obviously complicated as far as internal equipment is concerned, it presents the disadvantages inherent in multilayer reactors of a high cost, marginal reliability and rather difficult handling of the catalyst with a limited filling rate.